A wireless charging system using a magnetic induction phenomenon as wireless power transmission technologies wirelessly transmitting energy has been used.
For example, an electric toothbrush, a cordless razor, or the like, is charged by a principle of electromagnetic induction. In recent years, wireless charging products capable of charging portable devices such as mobile phones, PDAs, MP3 players, notebook computers, or the like, using the electromagnetic induction have been released.
However, the magnetic induction scheme inducing current through magnetic field from a single coil to another coil is very sensitive to a distance between the coils and a relative position of the coils to sharply degrade transmission efficiency even when the distance between two coils are slightly spaced or twisted from each other. Therefore, the wireless charging system according to the magnetic induction scheme may be used only in a short range of several centimeters or less.
Meanwhile, U.S. Pat. No. 7,741,734 discloses a method of wireless non-radiative energy transfer using coupling of resonant-field evanescent tails. The basis of this technique is that two same-frequency resonant objects tend to couple, while interacting weakly with other off-resonant environmental objects, which makes it possible to transfer energy farther away compared to the prior art magnetic induction scheme.
Even in the system wirelessly transmitting energy using resonance, wireless power is still transmitted to all of the receiving devices resonating with the transmitting device, which is not very effective.